While 0.12 ran butter smooth, 0.13 have a huge problem where some of heavy modified chunks (Huge buldings) cause huge lags when game loads them. FPS drops down from the perfect to 1-5 fps.
Flying far away from the chunks (Forcing chunk unload) doesnt solve performance issues, framerate stays same low until the map is reloaded.
Not all the big building lags, only some of them

If loading the map when player is in the chunk, loading takes lot of time, after the level loading game freezes and mcpe stops responding while the system works butter-smooth.